This invention relates to production of oil and gas wells, and particularly provides for a method for producing oil at a temperature adjustable from the formation temperature to a lesser temperature as desired.
Because of the normal decrease in temperature of produced crude oil from the temperature of the oil bearing formation, long chain paraffinic hydrocarbons, asphaltenes, and resins, which are often contained in crude oil, have a tendency to precipitate. This is caused by the cooling which occurs as such crude oil moves towards the surface of the wellbore being sufficient to cause precipitation of the paraffinic hydrocarbons and other components. Precipitation, of course, plugs the well and lines if the material is not removed at frequent intervals. Such removal requires the oil well to be shut in and treating methods to be performed to remove the precipitated hydrocarbons. The removal is done by running scrapers through the production tubing strings or treating the well with hot oil or solvents. These are known methods for treating oil wells to overcome the problems.
One technique disclosed in U.S. Pat. No. 3,456,735, of McDougall is a method of completing wells to prevent paraffin deposits. The method described requires the installation of a production packer between the casing and the production tubing string a short distance above the producing zone; next packer fluid is located within the annulus above the packer to equalize the pressure across it and prevent leaks; and then installation of rigid foam or similar porous material in the annulus surrounding the production tubing string and an outer tubing string or casing above the packer fluid. McDougall discloses several ways of obtaining the insulating material in the annulus surrounding the production tubing string including in situ formation.
The method of McDougall poses problems of maintaining the insulation when the production tubing string must be pulled for reworking the well and then reinstalled. Such procedure would require reinsulation of the production tubing string with additional foam producing material.
U.S. Pat. No. 1,901,141, issued to Battelle discloses a method of automatic oil well control to maintain the temperature and conditions of the hydrocarbon oils. Battelle utilizes a well construction that includes a casing set in a borehole extending below the oil bearing strata, a production tubing string within the casing which extends below the oil bearing strata near the bottom of the casing, and a pressure tubing string surrounding the production tubing string which extends to just below the oil bearing strata and is open to the oil bearing strata. The casing is perforated in the oil bearing strata and includes a packer above the oil bearing strata which prevents oil moving past the packer in the casing. The oil forms a pool in the casing below the perforations. Battelle requires a sealed, leak proof casing and a leak proof pressure tubing string surrounding the production tubing string. The pressure tubing string is connected at the surface to a pressure control valve which when activated by certain pressure settings will open or close a valve in the flow line leading from the top of the production tubing string to produce oil into a collection vessel. The pressure tubing string which surrounds the production tubing string is open to the crude oil and thus the pressure of the formation forces oil into the pressure tubing string until such time as the pressure has sufficiently built up to cause the pressure control valve to operate and open the production tubing string valve. Once the pressure in the pressure tubing string decreases sufficiently from production of crude oil, the pressure control valve will operate to close the production valve in the flow line leading from the top of the production tubing string. By this arrangement Battelle produces oil at a certain range of pressure and the pressure tubing string provides some measure of insulation surrounding the production tubing string. The casing above the packer does not have any offsetting pressure to retain the packer in place and to equalize the pressure across the packer to prevent leaks.
U.S. Pat. No. 3,763,935 of Perkins describes a method for thermally insulating the interior of one or more sections of casing, tubing, or other pipe and particularly in the permafrost zone of a wellbore. Perkins provides insulation by using closed annular zones in casing sections filled with a gas which will remain in the vapor state at the temperatures encountered in the wellbore. Where the section of casings are jointed he uses a solid insulating material surrounding the joint to provide insulation. The primary purpose of the Perkins method is to avoid damage of the permafrost zone around the well. This arrangement requires replacing the solid insulation whenever rework of the casing is required.
U.S. Pat. No. 3,685,583 of Phares describes another technique to protect the permafrost zone around an oil and gas well. Phares provides an atmospheric air flow passageway arranged to direct atmospheric air down along an outermost casing in the wellbore and return the air flow through the annulus between an inner tubing string or second casing and the production tubing string. In this arrangement cold air travels along a path next to the permafrost region to a distance therebelow and is then heated by the oil in the production tubing string near the bottom of the casing and flows upward around the production tubing string, thus the natural convection from heating the air permits the downward flow next to the permafrost region and return flow in the annulus between the production tubing string and the casing.